Reconstruction of winter climate variations during the 19th century in Japan

摘要

An attempt was made to reconstruct winter climate conditions in Japan for the period 1810/1811 to 1858/1859 on the basis of daily weather records documented in old diaries. Daily weather maps for each winter were drawn using 19th century weather records collected by our research group. Maps were divided into five types by classifying daily snowfall and rainfall distributions and the occurrence frequencies of each weather pattern for the period 1810/1811 to 1858/1859 were analysed. It was found that the occurrence frequencies of winter monsoon weather patterns were high from the late 1820s to the early 1840s. This period almost coincided with a summer cold period in the 19th century. The result implies that strengthening of a cold air mass around Japan occurred in the late 1820s, not only in summer but also in winter. The frequencies of the typical winter monsoon patterns correspond with the freezing dates of Lake Suwa, which have been used as an indicator of winter coldness in previous studies. On the basis of the frequencies of the winter monsoon weather patterns, mean January temperatures for western Japan were estimated. In the time series of estimated temperatures, a cooling period from the late 1820s to the early 1830s was revealed.